Volume Fresnel zone plates fabricated by laser direct writing

Abstract

A volume Fresnel zone plate and a method of producing such a device using a femtosecond laser for direct writing of zone plates. A volume zone plate has a number of Fresnel zone plate layers designed to focus light coherently to a single spot. Embodiments include both low numerical aperture (NA) and high NA zone plates, and provide a significant increase in overall diffraction efficiency over a single Fresnel zone plate.

Description

BACKGROUND OF THE INVENTIONRecently, the use of femtosecond laser direct writing to produce optical devices inside transparent materials has been growing considerably. Tightly focused femtosecond laser pulses can induce nonlinear absorption within the focal volume and permanently modify the index of refraction of the material, as discussed in the following papers: T. Tamaki, W. Watanabe, H. Nagai, M. Yoshida, J. Nishii, and K. Itoh, Opt. Express 14, 6971-6980 (2006); and A. M. Streltsov and N. F. Borrelli, J. Opt. Soc. Am. B 19, 2496-2504 (2002). Although the mechanism responsible for refractive index increase by ultrashort laser pulses is not fully understood, researchers have fabricated various optical devices inside transparent materials with this direct writing method such as waveguides, gratings and diffractive optical elements (DOEs). Examples are discussed in the following papers, which are hereby incorporated by reference along with all other references cited herein:A. M. St...

AI Technical Summary

Benefits of technology

The present invention provides a method of fabricating a number of Fresnel zone plates or modified Fresnel zone plates within a volume, referred to herein as volume zone plates, to achieve high diffraction efficiency. The volume Fresnel zone plate operates as a single diffractive optical element with a much higher diffraction efficiency than a single zone plate.

Problems solved by technology

Higher number of phase quantization levels provides higher efficiency, yet increases complication in fabrication.

The working principle of the Fresnel zone plate relies on the fact that the diffraction of light from alternating zones interferes constructively at the designed focal point.

However, all the previously studied zone plates suffered from low diffraction efficiency because of effects such as scattering from damaged regions, phase shift errors due to nonuniform index change inside fused silica, and planar fabrication.

Attempts to fabricate multi-level phase zone plates were also reported, yet had difficulty eliminating the phase shift errors due to fabrication.

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